Hi Ian,
You are making an assumption that when the lead screw is at a specific
point when the spindle is at an equally repeatable location. That's
essentially how the thread chasing dial works. If you were cutting a
thread that required engaging the half nut at any line it means that the
lead screw and spindle are linked in multiples of your lead screw pitch.
But if you are cutting a thread where the #3 position and only the #3
position is the right place it's possible that the spindle is 90 degree
offset for the #2 position, 180 for the #1 position and 270 offset for
the #4 position.
Now follow that same line of thinking for metric and you may find that
the nut would have to be engaged at the #1 position the first time. The
1.27 position the next time and so on.
John Dammeyer
Automation Artisans Inc.
Ph. 1 250 544 4950
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-----Original Message-----
From: 7x12minilathe@...
[mailto:7x12minilathe@...] On Behalf Of steam4ian
Sent: Wednesday, March 14, 2007 3:16 PM
To: 7x12minilathe@...
Subject: [7x12minilathe] THREAD CHASING
G'day all.
On another topic, John (born4something) and I have been discussing
thread chasing dials. If you have a metric lead screw you need a
range of pinions on the dial to suit different threads. This is
tedious. to cut an imperial thread (with metric lead screw) you have
to keep the half nut closed the entire time and reverse the lathe out
between cuts, also tediuos.
There are two common methods to keep the tool "in the groove" for
each pass, namely, using the thread chasing dial or reversing between
cuts.
I think there is a third method, does anybody know of it?
In the wee small hours of the night, my designing time, I dreamt up a
method. It involves using a saddle stop at the RH end of the cut
(assuming R to L cutting). At the end of each pass you dissengage the
half nuts, withdraw the tool and crank the saddle back hard against
the stop. You set you tool and then gently engage the half nuts by
feeling them into position. This last process would be made easier by
a circular clip on the lead screw to set indicate when the screw is
at the same point of rotation corresponding to the first engagement.
The gib for the half nuts would need to be well adjusted.
Comments and advice is sought, over to you.
One good turn deserves another. (Luke6:31 Machinists Bible)
Regards,
Ian
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